What is the passive voice of:
"I will convey my decision to you in a week"
- My decision will be conveyed to you in a week by me.
- You will be conveyed my decision in a week by me.
You may recognize that in your example sentence, "my decision" is the direct object and "to you" is the prepositional phrase that is the indirect object. More on direct and indirect objects of English verbs.
I will give you my answer tomorrow.
I will give my answer to you tomorrow.
Either way, "you" is the indirect object. When forming the passive it's most common to use the direct object, not the indirect object.
My answer will be given to you tomorrow (by me).
However, it's not unusual to use the indirect object with certain verbs like give:
You will be given my answer tomorrow (by me).
Similarly, in your example, either is possible. The problem is with the verb conveyed which sounds very formal. The image is that the decision will be physically transported by a messenger, as with an old-fashioned, formal communication from a high-ranking government official.
There's nothing wrong with this if that is what you want to say. Be aware, though, that almost any combination of this sentence may sound condescending, as if a person with power is sending their decision to a person with lesser power. Often, the more formal the language, the more condescending the nuance.
Again, nothing wrong if that is your intent -- but if not, then it would be better to rephrase it as a request:
Is it all right if I let you know in a week?
I can give you my decision in a week, if that's OK?
With requests, you would not use the passive voice. Also, it's probably better to avoid more formal verbs like convey and instead use send or give.